Electrostatic stepper motors use a plurality of set voltage patterns to provide controlled incremental movements often referred to as steps. Changing the voltage pattern moves the stepper motor through those steps, or to a resting position at a particular step. Electrostatic stepper motors are controlled by setting a voltage pattern on an array of stator electrodes that interact with the fields on an array of translator electrodes to produce a force (and motion) on the translator stage. This force is a function of the voltage pattern and the relative position of the translator electrodes with respect to the stator electrodes. Electrostatic stepper motors can be built to produce linear or rotational motion. The present invention was developed for a linear electrostatic stepper and certainly applies to rotational electrostatic stepper motors but may apply to magnetic stepper motors (linear or rotational) as well.
To summarize the description provided above, the resulting motion (or force) from a change in the voltage pattern depends not only on the applied voltage pattern but also on the relative location of the translator electrodes with respect to the location of the stator electrodes. However, if the voltage pattern is changed before the motor comes to rest at the location consistent with the previous state, the relative position of the translator electrodes with respect to the stator electrodes won't be known. Many applications for stepper motors limit the rate of change of the applied voltage pattern to prevent the translator and the applied voltage pattern from getting too far out of phase and possibly slipping into the adjacent potential well. For applications that require the motor voltage pattern to change before the motion has stopped, the output force is a nonlinear function of the voltage pattern and the relative position of the translator and stator electrodes. The present invention effectively linearizes the response of the motor for use in control systems with bandwidths that don't allow the motor to stop before issuing a new voltage pattern to the motor.
A linearized electrostatic stepper motor accepts a force request from the system controller and calculates the required voltage pattern to produce the required force, regardless of the present position of the motor. This is achieved by modeling the dynamic state of the motor. The model can be augmented for improved accuracy by measuring the present motor position, but this is not a requirement.
A linear motion electrostatic stepper motor can be configured as shown in FIG. 1. The translator stage is supported by springs that allow horizontal motion while maintaining the desired vertical spacing. The translator electrodes are parallel to the stator electrodes, which are mounted on the fixed position stator stage below the movable translator stage. Semiconductor fabrication techniques can be used to build linear motion electrostatic stepper motors that are physically small and can provide swift, extremely accurate, controlled motion.
A voltage pattern is applied to both sets of electrodes to generate a spatially alternating electrostatic field on each surface of the two facing electrode sets. Typically the translator pattern is fixed and the stator pattern is varied to control the motor motion. For a more complete understanding of such drive voltage patterns and the forces generated by an electrostatic surface micro motor, please refer to Electrostatic Surface Drives: Theoretical Considerations and Fabrication, Storrs Hoen et al., Agilent Laboratories, Jun. 16, 1997, incorporated by reference herein in its entirety.
Typically stepper motors are controlled such that the motor position is the control reference and care is taken to ensure that the motor position never slips too far out of phase with the intended position. Such control can be achieved through the use of predetermined acceleration/deceleration curves. In the present invention the relative positions of the translator and the stator is modeled, allowing the calculation of the required voltage pattern that produces the desired force output.
Motor dynamics play an important role in design of a motor control system. The dynamics of the motor are determined by its mechanical properties (mass and spring constants) and the forces generated by the applied electric fields. The mass/spring dynamics form a resonance response. The electric fields from the electrodes significantly increase the frequency of the motor resonance by generating a force that tends to hold the translator in place (with respect to the stator electrodes).
These electrical forces are a function of the field strength (voltage) applied to the stator/translator electrodes and the motor phase, i.e., the relative position of the translator plate with respect to the electric field generated at the stator electrodes. One effect that may impact the effective field strength for electrostatic motors is dielectric charging. Areas of charge can build up (especially in the presence of water vapor) in the motor insulators, which effectively cancel a portion of the applied external electric field. This reduces the electrical restoring forces of the motor and lowers the resonant frequency of the energized motor. Building a closed loop feedback controller that can adapt to the changing motor response is a more challenging task than building a static controller, but the task is still possible. For a good discussion on the problems associated with motor resonances, see U.S. Pat. No. 6,208,107 entitled “Use of Digital Current Ramping to Reduce Audible Noise in Stepper Motor,” to Maske et al., issued Mar. 27, 2001. Another patent that discusses motor resonances and provides a good technical discussion of stepper motors as well is U.S. Pat. No. 6,285,155 entitled “Pseudo Half-Step Motor Drive Method and Apparatus,” to Maske et al., issued Sep. 4, 2001.
The present invention allows the use of a fixed feedback controller designed to control the fixed mass/spring mechanical resonance of the motor in conjunction with a model of the motor that estimates the present position and allows the determination of the motor stator voltage pattern which will produce the requested output force. This approach effectively eliminates the electrical restoring force from the motor dynamics and eliminates the problems associated with the varying resonant frequency (since the variations were caused by the electrical restoring force, not the mechanical properties of the motor). Typical prior art controllers for a stepper motor monitor position error and produce the motor voltage state consistent with the desired location. In the present invention the motor is viewed as a force actuator and not a positional actuator. Once the motor has been effectively linearized, standard techniques are used to design a feedback controller which monitors position and requests a force output from the motor.
The preferred method of determining motor position is to develop a mathematical model of motor characteristics and motor dynamics to estimate motor position. Adding an external sensor to the model would enhance the accuracy of the model's estimating capabilities by providing positional data via feedback. To enhance the ability of the estimator model to provide accurate positional data, non-linear effects, such as motor saturation, could be included in the model.
A preferred embodiment of the present invention is a system for controlling the position of a linear motion electrostatic motor in a closed loop feed back system, the system comprising means for determining the position of the motor, means for converting positional error into an output driving force request, and means for calculating the motor voltage pattern which will produce the requested force at the present motor position.
There are several methods for determining the present motor position. Dead reckoning can be used if the motor response is well modeled and if any external forces are small. Feedback can be used to enhance the accuracy of the estimated position, essentially adding the measured position (which may be noisy) to the estimator model. In cases where the measured position is not noisy, the directly measured position can replace the estimated position.
In another preferred embodiment of the present invention, to minimize the effect of resonant frequencies changes associated with the variation of the electrical restoring force of the stepper motor, a position sensor mounted on the motor generates a signal which relates to the actual position of the motor. The data provided by the position sensor is then transferred to an analog-to-digital converter, to be outputted from this converter as digital counts which are inputted to a position estimator which combines the present data with the past history and known dynamics to estimate the current position of the motor. The mechanical position data is then mapped into the appropriate motor voltage pattern. This electrical equivalent of the mechanical position component provides a motor state offset, which, when combined with the force request input to the motor to complete the linearization circuit, effectively removes the positional dependence from the motor state.
Another preferred embodiment of the present invention is a method for controlling the position of a linear motion electrostatic stepper motor in a closed loop feedback system, wherein output force requests are delivered to the motor in succession to move the motor from a first position to a second position, the method including the sensing a positional change of the motor following motor movement in said successive incremental steps, converting positional error into a requested output driving force, adding the output driving force with a positional offset provided by a mathematical model, and delivering the motor voltage pattern which will produce the desired force offset force provided by the comparing means to a means for combining the offset force with the input driving forces, thereby to remove the mechanical component of linear movement from the input driving forces.